Without the certificate of authenticity that confirms the signature is valid, I think you may have to go the market place route. The issue with the value of these books is that it strongly depends on who is willing to pay that money for them. There are some comic books whose worth was estimated at thousands of dollars but they sold for a few hundred, because no one could be found who were willing to pay the full estimated amount. Maybe you can find Nicholas Cage, who may be willing to pay five times the estimated value, but most likely, you may need to settle for the highest bidder on EBay. Maybe start the bidding at $100-$150 and see if anyone bites.

You may also be able to sell them to a used bookstore - many vintage shops buy signed editions and are willing to do their own research into the market value before giving you an offer.